1. Field of the Invention
The present invention relates to a pigment dispersion and an ink composition using the ink composition, and more specifically, a pigment dispersion with good pigment dispersibilty and an ink composition using good color-forming characteristics using the ink composition.
2. Related Art
With regard to an image recording method for forming an image based on image data signals on a recording medium such as paper, there have been known image recording methods such as the electrophotographic system, the thermal transfer system such as a sublimation type and a fusion type, and the inkjet system and the like. Among them, the inkjet system can be put into practice with the use of relatively inexpensive machines, and an image is directly formed by jetting ink droplets on a recording medium only on the area where the image is required to be recorded, so that the ink can be effectively used, and further running cost is low, and noise is low, thereby the inkjet system is excellent as an image recording method.
Moreover, the inkjet system enables printing not only on plain paper, but non-water absorptive recording media such as a plastic sheet and a metal plate. However, the speeding up of printing and formation of high quality image prints are the most important issues, and the inkjet system is characterized in that the time required for drying and curing of printed droplets influence the sharpness of an image.
As one of the inkjet systems, there is a recording method using an ink composition for inkjet recording which is curable by irradiation with an actinic ray. According to this method, the curable ink composition is irradiated with an actinic ray immediately after a print is formed with the use of the curable ink composition to cure ink droplets thereof, so that a sharp image can be obtained.
In the curable ink composition which can be used for such a recording method, a high pigment dispersibility and the stability of the pigment dispersibility over time are required for forming a high definition image with an excellent color forming property. In general, it is essential to form fine particles of pigment in order to impart clear color tone and high coloration performance to an ink composition. In particular, in the inkjet recording ink composition, since jetted ink droplets greatly influence the sharpness of an image, it is essential to use the small quantity of droplets to be jetted and the particle in a smaller size of the ink composition relative to the thickness of a cured layer formed by the ink composition.
When the pigment particles are formed in a smaller size in order to obtain a high coloration performance, the dispersion of the pigments becomes difficult and pigment aggregates are easily formed. When a pigment dispersant is added so as not to form pigment aggregates, the problem of an increase in the viscosity of ink composition arises.
Both the occurrence of the pigment aggregates and the increase in the viscosity of an ink composition exert an adverse influence on the ink jetting property, thereby resulting in great decrease in the performance of the ink composition. Further, since the ink composition used for the inkjet system is accommodated in a cartridge, the ink composition is heated at the time of being jetted, and the temperature of the ink composition is lowered at the time of not being jetted and during being stored, the ink composition is subjected to reiterative changes in temperature between heating and cooling. The temperature change exerts an adverse influence on the dispersibilty of the pigment, in addition to the decrease in the dispersibility over time, there are problems such that thickening and agglomeration of the ink dispersion easily take place.
Under these circumstances, it has been demanded that an ink composition which has sufficient flowability, and contains finely dispersed pigment particles stably and further, has the excellent storability of a pigment dispersion over time, and to this end, various proposals on dispersants for obtaining stable pigment dispersions have been made.
For example, an ink composition in which a pigment derivative as a dispersant in order to enhance the affinity for a pigment is used (for example, refer to Japanese Patent Application Laid-Open (JP-A) No. 2003-119414 and JP-A No. 2004-18656), an ink composition in which a polymer having a basic group as a dispersant is used for specific pigments such as phthalocyanines and quinacridones (for example, refer to JP-A No. 2003-321628), and an ink composition in which an ink composition which contains a dispersant such as poly(ethyleneimine)-poly(12-hydroxystearic acid) graft polymer and a specific monomer which dissolves the dispersant, without using an organic solvent (for example, refer to JP-A No. 2004-13158), have been proposed.
However, although these pigment dispersions have an improved stability owing to the function of the dispersant as compared with conventional ink compositions, the formation of finely dispersed particles of pigments being used is still insufficient, and there is still a room for improvement on forming pigment fine particles, and further, there is a problem such that the dispersion stability is still insufficient after the ink composition is stored for a long period of time, or is subjected to reiterative changes in temperatures.
Moreover, it is required to satisfy various characteristics such that a nozzle is not clogged (jetting stability) as a radiation curable ink composition used for an inkjet recording method, in addition to the storability.
In addition, for the purpose of improving the pigment dispersibility, there has been proposed a pigment dispersion containing a dispersant in which an acridone moiety is introduced into the main chain skeleton (for example, refer to JP-A No. 2007-277514).